Conventional reflective image display technology utilizes two contrasting colored particles of opposite charge polarity dispersed in a liquid medium and contained within a microcapsule. The particles may be moved electrophoretically by application of a voltage bias to reflect or absorb incident light to display bright, dark and gray image states to the viewer. The electrophoretic two particle display provides diffused reflection very much like the viewing experience of traditional paper along with long battery life. However, the limited brightness and slow switching speed of the two particle display limits its use in applications such as web browsing and viewing of video content.
Light modulation in conventional total internal reflection (TIR) image displays may be controlled by movement of electrophoretically mobile particles into and out of the evanescent wave region at the surface of the front sheet. The front sheet may comprise of a plurality of structures such as convex protrusions of a hemispherical shape that are capable of total internal reflection of light. The front sheet typically further contains a transparent electrode layer. The rear sheet may include a rear electrode layer. An electrophoretic medium consisting of electrophoretically mobile particles comprising of a charge polarity suspended in a fluid is disposed between the front and rear sheets. An applied voltage moves the electrophoretically mobile particles through the liquid electrophoretic medium where they may be moved to the front sheet and into the evanescent wave region where TIR may be frustrated. Thus the display may be referred to as a frustratable total internal reflection (FTIR) display. The modulation of particles of only one charge polarity and the fact that TIR frustration happens within the evanescent region which is only about 0.5 μm thick, allows the switching speed of a FTIR display to be much faster than the traditional dual particle electrophoretic display technology described in the preceding paragraph. This is due to the fact that the particles in FTIR displays may only need to move a much shorter distance than particles in conventional two particle electrophoretic displays.
Dry toner particles developed for the electrophotographic industry are charged particles that move through air under the influence of an applied electric field. These types of particles may also be used in FTIR-based displays to further increase the rate of movement of the particles. Additionally, the maximum reflection (brightness) of a FTIR display is determined by the refractive index difference between the materials that constitute the front sheet and the medium that is in contact with the hemisphere film. Dry particle FTIR displays use a gas instead of liquid as the medium to carry the charged particles. Dry particles that are moved by an applied electric field in a gas are referred to as electrostatically mobile particles. Charge particles that are moved by an electric field in a liquid or fluid medium are referred to as electrophoretically mobile particles.
A gas provides maximum refractive index difference with the hemisphere TIR film thus yielding the maximum level of brightness the display may attain. At normal incidence, the reflectance R of a hemisphere of a transparent front sheet is given by equation (1):
                    R        =                  1          -                                    (                                                η                  m                                                  η                  h                                            )                        2                                              (        1        )            where ηh is the refractive index of a hemisphere of a transparent front sheet and ηm is the refractive index of medium adjacent the surface of a hemisphere of the front sheet where TIR occurs. Thus, if the hemisphere is formed of a higher refractive index material such as polycarbonate (ηh˜1.59) and if the adjacent medium is a fluorinated solvent such as Fluorinert (ηm˜1.27) with a lower refractive index, a reflectance R of about 36% may be attained. In comparison, if the medium is a gas (ηm˜1), a reflectance R of about 60% may be attained. From the calculation it can be determined that using a gas as the medium containing the particles may be a major advantage over conventional reflective displays. Furthermore, there is only one plurality of one type of particle present in the display. Thus, only the interaction of the plurality of one type of particle with the electrode surface and not with a plurality of particles of opposite charge polarity (other than small non-optically active counterions) and color must be considered when determining the display's optical performance.